No matter where we live there are raccoons fox coyote mink and the list goes on and flying preds. too. We have to just be vigilant when we let our ducks/ poultry out in our yards. I let mine be in as natural a habitat as I can give them. Not saying I haven’t lost any because I have (chickens) but we put up fencing to keep them from wandering too far and I am here almost all the time. That said doing all you can to protect and sounds like you have given them great security In their housing. So what can we do when we let them out is watch as close as we can. Is your pen secured over top so nothing can climb in that’s very important too.